The Supreme Instruments **Model 385 Automatic** is a 1930s-era emission tube tester from the Supreme Instruments Corp of Greenwood, Mississippi — one of the more elaborate Depression-era testers, with the company's signature 'Automatic' single-knob tube selection setup that replaces the messy pin-switch banks of competing units of the era. Emission testers measure only **whether the cathode still emits enough electrons** under a specified plate voltage — they do not measure Gm and therefore do not catch a tube that has lost transconductance while retaining emission.
Supreme Instruments was an active mid-tier US test-equipment maker from the late 1920s through the late 1940s, eventually folded as the post-war majors (Hickok, Triplett, Sylvania) consolidated the tube-tester market. The 385 is one of their most ambitious units. Pair it with the [Heathkit TT-1](../Heathkit%20TT-1%20Tube%20Tester/CLAUDE.md) (Gm tester) and the [eTracer](../eTracer/CLAUDE.md) / [uTracer6](../uTracer6/CLAUDE.md) (curve tracers) for the full triage spectrum. Common service issues on a unit this old: the original line cord, the original electrolytics, the selenium rectifier (replace), the meter movement (often needs cleaning + rezero), and the wax-paper bypass caps (universally need recap).
A multi-volume deep dive on this instrument is in planning. When the first volume lands in the source project's 02-inputs/volume_sources/, this page upgrades automatically — no website code change required.